As is known in the art, radio frequency (RF) and analog circuits are integrated with high-speed and high-performance digital circuits in CMOS mixed-signal applications. In this regard, conversion from analog to digital is generally done at lower base band frequencies, such as those on the order of approximately 1 MHz and lower. However, it is sought to execute the conversion from analog-to-digital at higher frequencies, such as RF to improve performance and simplify circuit design. Conversion at high frequencies requires capacitor elements that have a high quality factor (Q) and which exhibit linearity over the applied voltage ranges and over temperature ranges in which the circuit will be disposed.
Capacitors utilized in current CMOS technology do not adequately meet the needs for conversion at high frequencies. For example, typically electrodes of such capacitors are made from the bulk, monocrystaline silicon of the substrate, or polysilicon. The capacitance of such structures is dependent on two components, the insulator capacitance and the bias-dependent silicon surface depletion capacitance. As a result of the depletion capacitance associated with silicon-based electrodes, the capacitors display poor linearity.
To solve the problem of lack of linearity associated with silicon-based electrodes, electrodes may be made of a material containing metal (e.g., Ti, W), thereby limiting surface depletion capacitance. While capacitors having metal electrodes are generally known in the art, a capacitor that can be formed using current process flows has not been developed. Metal-electrode capacitors that may be fabricated with high precision, having a high "Q" are available, but require specialized process flows that are not cost effective.